During the constant-rate drying period of high-moisture agricultural produce placed in a convective hot air stream:
Options:
(A) Moisture evaporates from a continuous saturated liquid film at the product surface, and the surface temperature remains at the wet-bulb temperature of the drying air.
(B) Internal moisture diffusion governs the mass transfer rate, and product surface temperature approaches the dry-bulb temperature.
(C) The drying rate decreases linearly with decreasing moisture content down to equilibrium moisture.
(D) The rate of drying is completely independent of the drying air velocity and relative humidity.
Answer Key:
A
Step-by-Step Solution & Explanation
During the constant-rate drying period, water is transported from the interior to the surface by capillary suction at a rate equal to or exceeding the surface evaporation rate. The surface remains completely wet (saturated), behaving like a free water surface where convective heat supplied by air matches the latent heat of vaporization. Consequently, the surface temperature remains constant at the wet-bulb temperature ($T_{wb}$) of the air.